Radio navigation system



July 6,

E. J. isms-NER 2,444,445

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' lh l M 97A 97 |NvENroR P196 gai/U ER/,c d. A56/STER I v ev "M99 .999 Ark Patented July 6,1948

UNI 1f RADIO NAVIGATION SYSTEM Eric J. Isbister, Garden City, N. Y., assignor to The Sperry Corporation, a corporation of Dela- Application November 10, 1943, Serial No. 509,666

l ldclaima.

This invention relates, generally, to a radio navigation system in which the position of a craft is fixed by the intersection of hyperbolas. each of which represents the path of a point that maintains a constant difference in its distance from two fixed points. In a navigation system of this type the distance difference may be determined by measuring the time intervals between the arrival at the craft of signals synchronously transmitted from separate stations of known location.

The present invention particularly concerns receiving apparatus for use in such systems, whereby a measure of the difference in the distances of the receiver from two transmitting stations is automatically provided and the hyperbolas may thus be readily identified for fixing the position of the receiver.

The major object of the invention is to provide a radio navigation system for mobile craft having receiving apparatus for automatically determining data to readily fix the position of said craft.

Another object of the invention is to provide a radio navigation system for mobile craft having apparatus for automatically determining the difference in the distances between the craft and separated transmitting stations.

Still another object of the invention is to prov ide receiving apparatus in a radio navigation system, having means for automatically identifying and segregating signals from two separate stations.

A further object of the invention is to provide a radio navigation system having a receiving ap paratus for segregating signals received from separated transmitting stations and automatically measuring the time interval between the reception of energy from said stations.

A still further object of the invention is to provide a radio navigation system having a receiver for segregating signals received from separated transmitting stations, and automatically indicating the dierence in the distances of the receiver from said stations to identify hyperbolas whose intersection fixes the position of said receiver.

Other objects and advantages of the invention will become apparent in the following specification, taken in connection with the accompanyin drawings in which Fig. 1 is a diagram showing the relative kpositions of three transmitting stations to provide a clearer understanding of a radio navigation system embodying the present invention;

Fig. 2 is a diagram showing the time phase relationship of various signals in the system for convenience in explaining the operation of the invention, and

Fig. 3 is a schematic diagram of a receiver embodying the invention in its preferred form.

According to the invention signals are periodically transmitted from several separated stations in synchronized relation, and a receiver detects the signals from several stations. The detected signals of each station are identified and segregated from other received signals by selectors which are connected in certain pairs, to measuring devices for determining the time interval between the reception of signals from each of said pair of stations. The time intervals between the arrival at the craft of signals from the separate transmitting stations are automatically measured and represent a measure of the difference in the distances of the craft from the transmitting stations in each pair. By supplying signals from at least two diierent pairs of transmitting stations to the measuring devices, it is possible to locate the craft on'two hyperbolas whose intersection determines the position of the craft.

As shown in Fig. 1, three transmitting stations II, I2. and I3 may be employed to synchronously transmit signals to be used by the various mobile craft for fixing their respective positions. As is well known, the path of the point that has a constant difference in its distance from two fixed points describes a hyperbola. The three transmitters Ii, I'2 and I3 may be arranged in pairs to represent the foci of families of hyperbolas. Every position of the craft may be determined byy the intersection of two branches of the hyperbolas of each family.

For purposes of illustration, it is assumed that the craft is located at the point It. The dierence in the distances of thempoint Ii from the stations II and I2 locates the craft on a lhyperbola having branches I5 and I6. Similarly, the difference in the distances of the point I4 from stations I2 and I3 locates the craft on a hyperbola having branches I1 and I8. The particular branches of the hyperbolas on which the craft is located may be readily determined by ascertaining which station is the nearer. Since the point It is nearer to station Il than station I2 it is obviously located on the branch I5 of the hyperbola, and likewise, since it is nearer station I3 than station I2 it is located on branch I8 of the hyperbola.

Obviously, a map may be constructed showing a plurality of hyperbolas drawn with the stations II and I2, and' I2 and I3 as foci with their branches arranged in convenient increments to cover a desired range of distance differences. By determining the particular hyperbola of each family on which the unknown position of the craft is located and identifying the correct branch of each hyperbola, it is possible to fix the absolute position of the craft by locating the intersection of the hyperbolas branches on the map.

The three transmitters located at stations I I, I2 and I3, periodically transmit a group of short pulses of radiant energy. The three stations are synchronized so the groups of pulses radiated from each station have the same period. However, the time spacing of the pulses in each group is different for each station.

One arrangement of the-synchronized pulses is shown in the time chart in Fig. 2 where the groups of pulses radiated by stations II, I2 and I3 are shown at A, B, and C, respectively. -The pulses of energy from station II, as shown in Fig. 2A, include groups 2|, 2| composed of first and second pulses 22 and 23, respectively; the pulses from station I2 include groups 24, 24 composed of first and second pulses 25 and 26, respectively, and the energy radiated by station I3 includes groups of pulses 21, 21 composed of first and second pulses 28 and 29 respectively. The second pulse of each group is shown as being radiated simultaneously by all three stations. Since the spacing between the two pulses in each group is diierent for each station, the rst pulse appears at different times for the respective stations. In this case, the synchronism of the groups radiated is determined by the second pulse. Although the second pulses are shown as being radiated simultaneously by all of the stations, they may be radiated at any known time intervals from the various stations.

Since the distance of the craft determines the time required for energy to travel from the transmitting stations to the craft, it will be apparent that the difference in the distances of the craft from the transmitting stations corresponds to the time interval between the reception of pulses from said stations provided the pulses are radiated simultaneously. If they are not radiated simultaneously, allowance may be made for the known time interval between the radiation of the pulse groups by the several stations.

Considering for the moment only the second pulses 23, 26 and 29 radiated simultaneously by the stations II, yI2 and I3 respectively, it will be apparent that the time elapsing between reception of these pulses at the location of the craft will provide a measure of the difference in the distances of the raft from said stations, thereby locating the craft on hyperbolas. In order to determine on which branches of the hyperbolas the craft is located it is necessary to determine which transmitting station is nearer to the craft, that is, which pulse is received rst. Unless the rst received pulse can loe identified there are obviously several intersections of the branches of the hyperbolas which could represent the position of the craft.

By the present invention, the spacing ofV the various groups of received pulses is used to identify the pulses received fromeach station and to segregate them from other received signals. When this is done it is possible to supply pulses .from selected stations to a measuring device providing a measure of the time interval between the reception of the pulses from each station and also indicating which of the pulses is first received.

Although Fig. 1 shows only two hyperbolas. it will be apparent that a third hyperbola, having stations II and I3 as foci, may be used to check the position fixed by the intersection of the other two hyperbolas.

The receiving apparatus shown in Fig. 3 embodies the invention in its preferred form. Pulses of energy from the transmitting stations II, I2 and I3 are collected by an antenna 3I and supplied to a receiver 32 which may detect lthe pulses received from all three stations. Since the craft is located at point I4 the pulses from the various stations will be collected by the antenna 3I `at diiercnt times, depending upon the time intervals required for them to travel from the transmitting stations to the craft. The time rela tion of the received pulses is shown at A', B and C' in Fig. 2. 'Ihe time interval between the reception of the second pulses 23, 26 and 29, as shown at A', B' and C' in Fig. 2, provides a measure of the difference in the distances of the point I4 from the three transmitting stations II, I2 and I3.

The output of the receiver 32 is connected as fby a condenser 34 to a delay network 35 which may be formed of a section of artificial transmission line having appropriate inductive and capacitive elements 36 land 31 respectively. The delay network is provided with suitable taps as represented by leads 4I, 42, 43, 44, 45 and 46, for providing different time delay of received energy, These taps are connected to contacts of switches 41, 48 and 49 havingcontactcrs 5I, 52 and 53 settable to pick up signals detected by the receiver 32 and delayed for predetermined time intervals by the delay network 35.

The contactors 5I, 52 and 53 are each connected to control grids 54, 55 and 56 of selector tubes 51, 58 and 59 which may be of the pentagrid type. The other control grids 6I, 62, and 63 of the respective selector tubes are connected directly as by lead 64 to the receiver 32.

The selector tubes 51, 58 and 59 are arranged in circuits which are sometimes referred to as coincidence selector circuits. It should be understood, however, `that other types of selector circuits may be used without departing from the invention. Each of the selector circuits for tubes 51, 58 and 59 are identical, with the exception of the input circuits for supplying signals to the control grids 54, 55 and 56, as Will subsequently be explained.

Control grids 6I, 62 and 63 are connected through resistors 65, 66 and 61 to a source 68 of negative potential which provides a sufficient bias to normally cut off the selector tubes 51, 58 and 59. Cathodes 1I, 12 and 13 of the selector tubes 51, 58 and 59 are connected through suitable cathode resistors 14, 15 and 16 which are shunted in a conventional manner by by-pass condensers 11, 18 and 19. The screen grids of the tubes are connected through dropping resistors 8i, 82 and 83 to a source 60 of positive potential and are also connected through by-pass condensers 84, 05 and 36 to ground. The suppressor grids of each of the selector tubes 51, 58 and 59 are connected directly to their respective cathodes.

Plates 81, 88 and 9are connected through load resistors 9|, 92 and 93 to the source 80 of positive potential. The outputs of the tubes 51, 58 and 59 appearing across the load resistors 9|, 92 and 93 are coupled as by condensers 94, 95 and 96 to the contacts of switches IOI, |02, ID3, and I 04, having settable contactors |05, |06, I01 and |38 for optionally supplying the outputs of any.

pair of the selector tubes to a pair of devices III, ||2. The devices and ||2 provide a measure of the time interval between the reception of pulses from each of the stations as well as an indication as to which pulse is rst received which corresponds to the difference in the distances of the receiver from the respective transmitting stations.

The taps of the delay network 35 may be arranged to correspond to the time spacing of the pulses in each group radiated by selected transmitting stations. For example, the tap represented by lead 43 may be arranged in the' delay network 35 so energy from the receiver 32 is de- `layed by a time interval corresponding to the time-between the radiation of first and second pulses 22 and 23 of the group 2 transmitted by station Il. This delay causes the iirst pulse 22 to appear on the lead 43 at the same instant as the second pulse 23 appears on the lead 64.

If the settable contactor 5| of the switch 41 is connected to the lead 43 it will supply the rst pulse 22to the control grid 54 of the selector tube 51,-at the same instant as the second pulse 23 is supplied by the lead 34 to the control grid 6|. 'I'he source 68 of negative potential is so selected that the selector tube 51 is normally biased to cut ofi and will pass current through its plate circuit, when positive pulses appear simultaneously on the two grids 54 and 6|.

In the event the second pulses are not transmitted simultaneously, the connections to the control grids 6 I 32 and 63 may include delay net- Work to delay pulses applied to one or more of them according to the time between transmission' of groups of pulses by the respective stations. In this case, the taps of delay network 35 will be adjusted to allow for the same time interval so first and second pulses from each station will be applied simultaneously to twogrids of one of the selector tubes.

It will be apparent from this description of the circuit for selector tube 51 that an output signal will appear across load resistor 9| only when pulses are detected by the receiver 32 which have a time' spacing corresponding to the delay interval determined by the tap in delay network 35, which is connected to the control grid 54. In the present case this delay interval corresponds to the time interval between the radiation of the rst and second pulses 22 and 23 by the transmitting station II. Any pulses or other signals having a different time phase relation, which may be collected by antenna 3| and detected by receiver 32, will be appliedat different times to the control grids 54 and 3|, hence they will not produce an .output signal as represented by the flow of current in the circuit of tube 87 through load resistor 3|.

In a corresponding manner the control grids 55 and 55 of the selector tubes 58 and 53l may be connected by appropriately setting contactors 52 and 53 of switches 43 and 49 to taps of the delay network 35, which represent a delay interval corresponding to the time interval between first and second pulses 25 and 26, radiated by transmitting station |2, and nrst and second pulses 28 and 23 radiated by transmitting station I3. As shown in the drawings the taps of the delay network 35 having delay intervals corresponding to the time interval between the spacing of the pulses in the groups radiated by stations I2 and I3 are represented by leads 44 and 45. Since the contacter 52 is set to connect the control grid 33 of selector tube 33 to the lead 44,4 current flows in the circuit of plate 33 and through the load resistor 32 to provide an out put signal only in response to the reception of pulses having a time spacing corresponding to the first and second pulses 23 and 26 radiated by the transmitting station I 2. Hence an output signal from the tube 53 is produced only in response to the reception of pulses from transmitting station I2.

Similarly, the connection of control grid 53 of selector tube 53 by the contacter 53 to lead 33 will cause arr output signal to appear across load resistor 93 in the circuit of plate 33 only in response to the reception of pulses spaced accord ing to the spacing of first and. second pulses 33 and 33 from' the transmitting station I3.

In this manner the output of each of the selector tubes is caused by the simultaneous application of the second pulse directly from receiver 32 to one control grid and the first pulse delayed by the network 35 to coincide with the second pulse. Therefore, the output signal from the selector tubes corresponds in time to the reception of second lpulses from the transmitting stations I I, I|2 and I3 by the receiver 32.

The contactors |35 and |33 oi the switches I3! and |32 are connected as by leads |I5 and |I6 to supply output signals from any two of the selector tubes to measuring device I I. Similarly, contactors |3'| and |33 of the switches |33 and |34 are connected asby leads ||`i and |I3 lor supplying output signals from a diierent pair of the selector tubes, according to the setting of the contactors |31 and |33 to the measuring device H2,

The measuring devices and ||2 are identical, so a description o the circuit of one of them will sumce for both. The same reference numerals have been applied to corresponding elements of both circuits.

'I'hese measuring devices each include a pair of tubes |2| and |22, and are arranged in a switching circuit similar to that sometimes referred to as an Eccles-Jordan circuit. Plates |23 and |24 of the tubes |2| and |22 are connected through resistors |25' and |23 to a suitable source |21 of positive potential. Grid |23 of the tube f|2| is connected through resistor |3| to plate |23 and through a resistor |32 to ground. In this manner a selected portion of the voltage of plate |23 is applied to the grid |23.

Similarly, grid |23 of the tube |22 is connected through a resistor` |33 to the plate |23 and through a resistor |34 to ground, :whereby a portion, depending upon the ratio of resistors |3| and |33, of the voltage of the plate |23 is applied to the grid |23. Cathodes |36 and i3? ci the tubes 52| and |22 are connected through windings |23 and |33 of a differential meter |4I having a pointer |42 which is deected according to the difference in the currents through the windings |33 and |33. The circuits of the cathodes i 36 and |37 are both completed from the windings of the meter 4| through a cathode bias resistor |43 to ground.

When one of the tubes |21 or |22 is conducting current, the other tube is cut off. For example,-

at A', B', and C',

'signal 91 from the to introduce a cut-ofi voltage .between grid I20 and cathode |31 of the tube |22` If a negative pulse is now applied to the grid |28 of suiiicient voltage to cut off the tube I2I, its plate voltage will rise, resulting in an increase in the voltage of grid |29 so the tube |22 will begin to conduct. When. this occurs. the voltage of plate |25 of the tube I 22 drops and the reduced` voltage applied from the plate |24 to the grid |28 of the tube I2| maintains the tube I2| in cut-olf con dition, although the negative -pulse is immediately removed from the grid |20. 'lihe circuit will remain in this condition until the tube |22 is cut olf as by the application of a negative pulse to its grid |29.

The grids |20 and |29 0f the tubes I2I and |22 are also connected to leads IIE and H5, respectively, which supply signals from switch contactors I 09 and |05 to the switching tubes I 2| `,and |22, respectively,

When lthe switch contactors I 95 and |06 are set, as shown in Fig. 3, output signals from the selector tubes 51 and 59 are supplied to the grids |29 and |20 of the tubes |22 and I2I, respecn tively. If the receiver is located at the point Ii (Fig. 1), signals from the transmitting stations I2 and I3 are received in the sequence shown of Fig. 2, and output signals are produced corresponding to negative pulses 91, 98 and 99, shown at A", B", and C", in Fig. 2.

A negative pulse corresponding to the output signal 91 will rst be supplied from selector 51 through switch IOI to cut-oir tube |22, and then a negative pulse, corresponding to output signal 98 will lbe supplied from selector tube 58 through switch |02 to cut-off tube I2| at which time the tube |22 will begin to conduct. The measuring circuit will remain in this condition until another output signal from the selector tube 51, corresponding to the reception of a later group oi pulses 2| from the. transmitting station il, cuts (siii Ehe tube |22 and the tube |2I begins to con- It will be, apparent that current ilows through the winding |38 of the differential meter lill for a time interval proportionate to the time interval between signals 91 and 98 from the selector tubes 51 and 59, respectively, whereas current will iiow through the winding |39 for a time interval proportionate to the time interval between signal 98 from the selector tube 58, and a subsequent selector tube 51 which. corresponds to the reception of a later group of pulses from the transmitting station I I.

Since the pointer |42 is responsive to the difference in the currents through |39 and |39, it will be deflected in a direction dependent upon whether signals from transmitting station II or transmitting station I2 are rst received by receiver 32. Full scale deflection of the needle 'will be obtained when the signals from the two stations are received substantially simultaneously, because, under these conditions one of the tubeswill conduct current for only a very short interval, and in the case of simultaneous reception of the signals should not conduct at all.

However, as the time interval between signals, received from different stations increases the magnitude of the deection of the pointer |39 is reduced until it reaches a central position. This will occur when the time interval between the reception of the signals from the two stations is equal to one-half of the time period -between successive groups of pulses transmitted lby the stations. For this reason it is desirable to have lconnects selector tube the period of radiation of groups of pulses such that the time between successive groups will be slightly more than double the time difference for the reception of signals from two of the stations in the most extreme position of the craft that can be anticipated. If desired the circuit of the meter may .be modified so the magnitude of the deflection of the needle |42 will be directly instead of inversely proportional to the difference in the distances to the two stations.

As already stated the circuit of the measuring device I i2 is identical with that of III. Leads Ill and H0 supply signals to the grids |28' and |29 of the tubes I2I and |22'. When the contactors |01 and |09 are set as shown in Fig. 3,

the measuring device I|2 responds to the diierence in the distances of the receiver from transmitting stations I2 and I3, since contactor |08 50 to the measuring device, and contacter |01 connects selector tube 59 thereto.

The diierential currents through windings off differential meter I4I, as determined by the time interval between the'reception of signals from transmitting stations I2 and I3, deflect pointer |92 to indicate the difference in the distances of the pointer II4 from the stations !2 and i3. The direction of the deflection ci the needle |62' indicates .whether a signal from the station II2 or that from station I3 is received first.

Contactors |05 and |06 may be set to connect any pair of the selector tubes 51, 58 and 59 to the measuring device III to thereby indicate the difference in the distances of the receiver from any two of the transmitting stations II, I2, and i9. As shown in the drawings the measuring device i i I provides a measure of the diiierence in the distances of the receiver, and the craft, from the stations Ii and i2. This distance is indicated by the position of needle |42 to thereby locate a craft on one of the branches i5 or I6 of a hyperboia. The particular branch I5 is determined by the direction of the deflection of the needle |42.

Contactors |01 and |99 of the switches |03 and |99 may likewise be set to connect the output of any two of the selector tubes 51, 58 or 59 to the measuring device H2. In this manner the switci-ies may be set so the measuring device II 2 will provide a measure of the diierence in the distances of the craft from any two of the transmitting stations II, I2 or I3, thereby locating the craft on a particular hyperboia. As shown in Fig. 3, the switches |03 and |04 supply signals from selector tubes 58 and 59, to the measuring device IIZ, which therefore provides a measure of the diiierence in the distances of the receiver 32 from thestransmitting stations I2 and I3. The direction of the deflection of the needle |42 indicates that the signals from station I3 are received first, thereby locating the craft on branch I 9 of the hyperbola.

In operation the contactors 41, 40 and 49 may be adjusted to supply inputs for selector tubes 51, 59 and 59 which will identify any three selected. stations, of which there may be a great number located throughout the world. Signals from these three stations II, I2, and I3 are identied and segregated by selector tubes 51, 59 and 59 respectively so the outputs of these tubes correspond only to the reception of signals from the particular set.

The two measuring devices III and II2 have their inputs connected through switches which station for which their inputs are may be set to supply output signals from any two of the selector tubes to their respective measuring devices. These devices then provide a measure in the difference in the distances of the receiver from the two selected transmitting stations which actuate each of them. With this information it ispossible to identify the hyperbolas defined by the distance differences and also to identify the particular branch of said hyperbolas on which the craft is located. Since two branches are identified their intersection xes the position of the craft. This automatic identiiication of branches of two hyperbolas on which the craft is located, enables a'navigator to readily locate the intersection of the two identified,

branches of the hyperbolas on a map which may be provided with families of hyperbolas for various pairs of transmitting stations as suggested above.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all .matter -contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a radio navigation system for a mobile craft. apparatus for determining the difference in the distances between said craft and a pair of transmitting stations radiating spaced pulses of electromagnetic energy comprising a receiver, a pair of selectors connected to said receiver and responsive to the spacing of transmitted pulses for segregating pulse signals of each of said transmitting stations from other received signals, and current reading means actuated by said selectors for providing a measure of' the difference in the `distances of said craft from said transmitting stations.

2. In a radio navigation system for a mobile craft, apparatus for determining the difference in the distances between said craft and a plurality of transmitting stations radiating spaced pulses of electromagnetic energy comprising a receiver, a plurality of selectors connected to said receiver and responsive to the spacing of transmitted pulses for segregating the pulses of each of said stations from other received signals, means responsive to a pair of said selectors for providing a measure of the difference in the distances of said craft from a pair of said transmitting stations, and a switch for selectively connesting said means to any pair of said selectors for measuring the distance difference to any pair of said transmitting stations.

3. In a radio navigation system for a mobile craft, apparatus for determining the difference in the distances between said craft and a plurality of transmitting stations radiating spaced pulses of electromagnetic energy comprising a receiver, a plurality of selectors connected to said receiver and responsive to the spacing of transmitted pulses for segregating pulses of each of said stations from other received pulses, each of said selectors having settable means for adjusting it to respond to the signals from a specific station, means adapted to be actuated by a pair of said selectors for providing a measure of the difference in the distances of said craft from a pair of said transmitting stations, and a switch for selectively connecting said means .to any pair of said selectors for measuring the distance difference to any pair of said transmitting stations.

d. In a radio navigation system having a plurality of separated stations for transmitting groups of diiferently spaced signal pulses, said groups having the same periodicity, apparatus for identifying the pulses of each station com- -receiver and a plurality oi. selectors connected to said receiver and responsive to the spacing of said pulses for segregating the pulse signals of each .of said stations from other received signals, each of said selectors having a settable means for adjusting it to respond to the spaced pulses from a specific station.

6. In a radio navigation system having a plurality of separated stations for transmitting groups of diilerently spaced pulses, said groups having equal periodicity, apparatus for determining the position of a mobile craft comprising a receiver, a plurality of selectors connected to said receiver responsive to the spacing of said pulses for segregating pulses of each of said stations from other received pulses, and current reading means actuated by said selectors for providing a measure of the difference in the distances of said craft from a pair of stations.

7. In a radio navigation system having a plurality of separated stations for transmitting groups of differently spaced pulses, said groups having equal periodicity, apparatus for determining the position of a mobile craft comprising a receiver, a plurality of selectors connected to said receiver responsive to the spacing of said pulses for segregating pulses of each station from other received pulses, and current reading means actuated by different pairs of said selectors for determining the difference in the distances of said craft from corresponding pairs oi said stations.

8. In a radio navigation system having a plurality of separated stations for transmitting groups of differently spaced pulses, said groups having equal periodicity, apparatus for determining the position of a mobile craft comprising a receiver, a plurality of selectors connected to said receiver responsive to the spacing of said pulses for segregating the pulses of each station from other received pulses, means responsive lte a pair of said selectors for determining the difference in the distances of said craft from a pair of said transmitting stations and an indicator actuated by said means for indicating said distance differences.

9. In a radio navigation system having a plurality of separated stations for transmitting groups of differently spaced pulses, said groups` having equal periodicity, apparatus for determining the position of a mobile craft comprising a receiver, a plurality of selectors connected to said receiver responsive to the spacing of said pulses for segregating the pulses of each station from other received pulses, separate means connected to different pairs of said selectors for determining the difference in the distances of said craft from certain pairs of said transmitting stations, and indicators actuated by each of said means for indicating said distance dierences and the nearest station of each oi said pairs.

assesses lll 10. 111 a radio navigationA system ioi a mobile craft, apparatus for automatically providing data for locating the craft by the intersection oi' branches of two hyperbolas having as their foci a plurality of stations for synchronously transmitting a plurality oi differently spaced pulses comprising a receiver, a plurality of selectors connected to said receiver and responsive to the spacing of said pulses for segregating pulses of each of said transmitting stations from` other received pulses, and means actuated by pairs of said seleotors for automatically determining branches of said fhyperbolas on which said craft is located.

11. In av radio navigation system for a mobile craft, apparatus' for automatically providing data for locating the craft by the intersection branches of two hyperbolas having as their foci a plurality of stations for synchronously transmitting a plurality of differently spaced pulses comprising a receiver, a plurality of selectors connected to said receiver and responsive to the spacing of said pulses for segregating pulse signals of each of said transmitting stations from other received pulses, means actuated by pairs of said selectors for automatically determining branches of said hyperbolas on which said craft is located, and indicating means actuated by said last-named means for identifying said branches of the hyperbolas Whose intersection xes the location of said receiver.

12. In a radio navigation system having a pair of separated stations for synchronously transmitting a plurality of differently spaced pulses, current reading apparatus for identifying the Apulses of each station comprising a receiver and a pair of selectors connected to said receiver and responsive to the spacing of said pulses for segregating pulses of each of said stations from other received pulses.

i3. in a radio navigation system having a pair of stations transmitting a plurality of differently spaced pulses, apparatus for determining the difference in the distances between a mobile craft and said stations comprising a pair of selectors at said craft responsive to the spacing of said pulses for segregating pulses of each of said stations from other received pulses, and means actuated by said selectors for providing a measure of the difference in the distances of said craft from said stations.

14. In a radio navigation system having a pair of stations transmitting a plurality of differently spaced pulses, apparatus for determining the difference in the distances between a mobile craft and said stations comprising a pair of selectors at said craft responsive yto the spacing of said pulses for segregating pulses of each of said stations from other received signals, means actuaterl by said selectors for determining the dif-l ference in the distances of said craft from said transmitting stations, and an indicator responsive to said means for indicating said difference.

ERIC J. ISBISTER.

REFEREN CES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 2,l/i i,203 Shanklln Jan. 17, 1939 2,198,113 Holmes Apr. 23, 1940 2,199,634 Koch May 7, 1940 2,204,432 Neufeld June 11, 1940 2,266,401 Reeves Dec. 16, 1941 2,328,944 Beatty Sept. 7, 1943 2,403,600 Holmes July 9, 1946 2,406,970 Smith' Sept. 3, 1946 

